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Aluminum Foil Fire Trick
Name: Ronald B.
Status: other
Grade: other
Location: CA
Country: USA
Date: Summer 2009
Question:
Some years back, and acquaintance used to show his friends
a trick using a piece of aluminum foil, such as from a gum wrapper,
folded to about 1 square centimeter in size. He would dip it in a
glass of tap water, place it in his palm and rub it a little, them
offer anyone watching to place it in their palm. After a few
seconds, the recipient would have to tip the aluminum off his palm
because it was getting too hot, and if kept there would result in a
serious burn. The little square of aluminum would continue to heat
up and eventually would be only a small pile of material resembling
ashes. The trickster would never reveal how this was done. Any ideas?
Replies:
The "trick" is that your acquaintance did not dip the aluminum gum wrapper
into water, but rather, into an alkaline solution. Aluminum is an amphoteric
metal, that is, it reacts both with acid and base. The heat of reaction of
aluminum with base (hydroxyl ion) is about 9000 cal per gram of aluminum
metal. That is a hugh amount of heat given that the heat capacity of a gum
wrapper is very small. So the magician passes the gum wrapper to the
unsuspecting volunteer when it begins to be uncomfortable. The volunteer
gets to feel the full force of the increase in temperature and cannot hold
the wrapper very long. What is left is a mixture of the paper and aluminum
hydroxide. I predict that some foaming was also observed -- hydrogen gas.
The reaction is: Al + (OH)^-1 + 3 H2O = [Al(OH^-1)4]^-1 + 3/2 H2(gas)
A "chemical trick" unmasked!
Vince Calder
Hi Ronald,
Both water and oxygen can react with aluminum to produce large
quantities of heat. The reason gum wrappers do not do so normally is
that a small layer of oxide already exists on top of the pristine
aluminum and this oxide layer prevents further contact with oxygen
and/or water. By rubbing the foil vigorously, the layer of oxide is
rubbed off and metallic aluminum (which was underneath) can now come
into contact with either water or oxygen. The reaction of aluminum
with oxygen is sped up at higher temperatures, so when the reaction
begins and starts producing heat, it actually speeds up - further
increasing the temperature and heat released.
Greg (Roberto Gregorius)
Canisius College
Hi Ronald,
My initial thought on your question was that something else must be
involved. Aluminum foil dipped in water, regardless of folding or
rubbing, should have this reaction. Neither simple foil nor those
alloyed with or bonded to different commercial consumer products
should do this. So something else must be involved. My guess is
that it is something that reacts with the water or the foil directly.
I looked a little further and found what is probably the substance
used. It seems that for some time people have been doing this trick
using mercury bichloride, HgCl2. The substance reacts with the
aluminum foil and produces the heat. The water is probably just a
ruse, or at best is something that can be used to adjust the time
over which the reaction proceeds. So your trickster would be
putting a little of the mercuric chloride in the aluminum foil
without your knowledge. The chemical used to be sold by magic
shops, but due to its toxic nature is no longer available (I believe
even illegal in the US). The mercury chloride can be adsorbed into
the body with the potential for chemical burns and other serious
health problems. (So do NOT try this!)
cheers,
Michael Pierce
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